Why do (at least the big US carriers) limit the award tickets and then require double the usual miles to get a ticket?

I know the obvious answer, but it really doesn't make sense. If they truely intend to honor the awards, then the seat will eventually be used by an award. So maybe they found a way to half the value (by charging double points) to get an award ticket.

Really, if a seat is available, it should be always be available for a standard award.

I like the "low cost" US airline FF programs much better in this case. For example, on Southwest, as long as a seat is open, it can be used by a standard Rapid Reward. They don't try to screw their supposedly most loyal, frequent flyers by this practice.